Multi-carrier based transmission techniques for satellite systems

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Report ITU
-
R S.
2173

(
07
/
2010
)


Multi
-
carrier based transmission


techniques for satellite systems







S Series

Fixed
-
satellite service







ii

Rep.

ITU
-
R S.2173

Foreword

The role of the Radiocommunication Sector is to ensure the rational, equitable, efficient and economical use of the
radio
-
frequency spectrum by all radiocommunication services, including satellite services, and carry out studies without
limit of frequency range on the basis of which Recommendations are adopted.

The regulatory and policy functions of the Radiocommunicati
on Sector are performed by World and Regional
Radiocommunication Conferences and Radiocommunication Assemblies supported by Study Groups.


Policy on Intellectual Property Right (IPR)

ITU
-
R policy on IPR is described in the Common Patent Policy for ITU
-
T/ITU
-
R/ISO/IEC referenced in Annex 1 of
Resolution ITU
-
R 1. Forms to be used for the submission of patent statements and licensing declarations by patent
holders are available from
http://www.itu.int/ITU
-
R/go/patents/en

where the Guidelines for Implementation of the
Common Patent Policy for ITU
-
T/ITU
-
R/ISO/IEC and the ITU
-
R patent information database can also be found.




Series of
ITU
-
R Reports

(
Also available online at
http://www.itu.int/publ
/R
-
REP/en
)

Series

Title

BO

Satellite delivery

BR

Recording for production, archival and play
-
out; film for television

BS

Broadcasting
service (sound)

BT

Broadcasting service (television)

F

Fixed service

M

Mobile, radiodetermination, amateur and related satellite services

P

Radiowave propagation

RA

Radio astronomy

RS

Remote sensing systems

S

Fixed
-
satellite service

SA

Space
applications and meteorology

SF

Frequency s haring and coordination between fixed
-
s atellite and fixed s ervice s ys tems

SM

Spectrum management




Note
: This ITU
-
R Report was approved in English by the Study Group under the procedure detailed


in Resolution ITU
-
R 1.



Electronic Publication

Geneva, 201
1



ITU 201
1

All right s reserved. No part of t his publicat ion may be reproduced, by any means what soever, wit hout writ t en permission of IT
U.



Rep.
ITU
-
R S.2173

1


REPORT
ITU
-
R
S
.
2173

Multi
-
carrier based transmission

techniques for satellite systems

(Questions ITU
-
R 46
-
3/4 and ITU
-
R 73
-
2/4)


(2010)


TABLE OF
CONTENTS


Page

1

Introduction
................................
................................
................................
......................


7

2

Applications and scenarios

................................
................................
..............................


8

2.1

High definition television/Three
-
dimensional television
................................
.....


8

2.2

Mobile multimedia

................................
................................
...............................


9

2.3

Broadband Internet

................................
................................
..............................


10

3

Satellite systems examples

................................
................................
..............................


11

3.1

17/24 and 21/24 GHz BSS systems

................................
................................
.....


11

3.2

Integrated MSS systems

................................
................................
......................


12

3.3

Ka
-
band broadband systems

................................
................................
................


13

4

System implementation methods

................................
................................
....................


14

4.1

Single and multi
-
beam satellite systems
................................
..............................


14

4.2

Digital satellite transmission system

................................
................................
...


15

5

Multi
-
carrier and multiple access systems

................................
................................
......


17

5.1

Basics of
multi
-
carrier transmission

................................
................................
....


17

5.2

Multi
-
carrier transmission over a satellite link

................................
....................


19

5.3

Multi
-
carrier based multiple access schemes

................................
......................


19

6

Peak
-
to
-
average power ratio reduction technologies
................................
.......................


20

6.1

Introduction

................................
................................
................................
..........


20

6.2

Peak
-
to
-
average power ratio
reduction techniques

................................
..............


21

6.3

CI
-
OFDM

................................
................................
................................
...........


23

6.3.1

Introduction

................................
................................
...........................


23

6.3.2

CI
-
spreading technology

................................
................................
.......


23

6.4

Power amplifier linearization: a technique to reduce the effect of PAPR

..........


25

2

Rep.

ITU
-
R S.2173



Page

7

Channel coding techniques

................................
................................
..............................


25

7.1

Channel coding

................................
................................
................................
....


25

7.2

Concatenated
codes
................................
................................
.............................


26

7.2.1

Single
-
level concatenated codes

................................
..........................


27

7.2.2

Multi
-
level concatenated codes
................................
............................


27

7.3

Turbo codes
................................
................................
................................
.........


29

7.3.1

Introduction

................................
................................
...........................


29

7.3.2

Convolutional turbo codes

................................
................................
...


30

7.3.3

Block turbo codes
................................
................................
.................


31

7.3.4

Methods for decoding turbo codes

................................
.......................


34

7.4

Low
density parity check codes

................................
................................
..........


37

7.4.1

Introduction

................................
................................
...........................


37

7.4.2

Description

................................
................................
............................


38

7.4.3

Graphical representation of LDPC matrices

................................
.........


38

7.4.4

Decoding LDPC codes: belief propagation
................................
...........


40

8

Link rate adaptation

................................
................................
................................
.........


43

8.1

Constant coding and modulation

................................
................................
.........


43

8.2

Adaptive

coding and modulation

................................
................................
.........


43

8.3

Hybrid ARQ

................................
................................
................................
........


44

9

Standards and transmission methods
................................
................................
...............


47

9.1

DVB
-
S
................................
................................
................................
.................


47

9.2

DVB
-
S2
................................
................................
................................
...............


48

9.3

DVB
-
RCS

................................
................................
................................
...........


51

9.4

DVB
-
SH

................................
................................
................................
..............


52

10

Performance parameters and models
................................
................................
...............


53

10.1

Performance and spectral efficiency of a multi
-
carrier
satellite system in
linear channels
................................
................................
................................
......


54

10.2

Evaluation of CI
-
OFDM transmissions in a non
-
linear satellite channel

............


61

10.2.1

System model
................................
................................
........................


61

10.2.2

Test results

................................
................................
............................


64


Rep.
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-
R S.2173

3



Page

10.3

Performance and spectral efficiency of CI
-
OFDM in a non
-
linear satellite
chann
el

................................
................................
................................
.................


65

10.3.1

System model
................................
................................
........................


65

10.3.2

Test results

................................
................................
............................


66

10.4

Performance of MC
-
CDMA in a non
-
linear satellite channel

.............................


74

10.4.1

System model
................................
................................
........................


74

10.4.2

BER performance of non
-
linear MC
-
CDMA satellite system
..............


74

10.4.3

Adaptive operation of MC
-
CDMA
satellite system

.............................


7
8

11

Future trends (on
-
board processing)

................................
................................
................


80

11.1

Introduction

................................
................................
................................
..........


80

11.2

Signal regeneration

................................
................................
..............................


80

11.3

Reducing latency: IP
-
routing and caching

................................
...........................


83

11.4

Flexible signals, flexible design: variable data rates, cross
-
layer optimization
and s
oftware
-
defined radio
................................
................................
..................


84

11.5

Implementation considerations and examples

................................
.....................


85

12

Conclusions
................................
................................
................................
......................


8
6

13

References

................................
................................
................................
.......................


86





4

Rep.

ITU
-
R S.2173


Abbreviations

3DTV



Three
-
dimensional television

ACK



Acknowledgment message

ACM



Adaptive coding and modulation

AM/AM


A
mplitude
-
to
-
amplitude

AM/PM


A
mplitude
-
to
-
phase

APSK



Amplitude and phase shift keying

ARQ



Automatic repeat request

ATC



Ancillary terrestrial component

AVC



Advanced video coding

AWGN



Additive white Gaussian noise

BCH



Bose
-
Chaudhuri
-
Hocquenghem

B
ER



Bit error rate

B
-
GAN



Broadband global area network

BLER



Block error rate

BPA



Belief propagation algorithm

BPS



Bent
-
pipe satellite

BPSK



Binary phase shift keying

BSM



Broadband satellite multimedia

BSS



Broadcasting
-
satellite service

BTC



Block turbo code

CCDF



Complementary cumulative distribution function

CCM



Constant coding and modulation

CDM



Code
-
division multiplexing

CGC



Complementary ground component

CI
-
OFDM


Carrier interferometry orthogonal frequency
-
division multiplexing

CN



Core network

CNR



Carrier
-
to
-
noise ratio

COFDM


Coded orthogonal frequency
-
division multiplexing

CP



Cyclic prefix

CPA



Chase
-
Pyndiah algorithm

CRC



Cyclic redundancy check

CTC



Convolutional turbo codes

DAB



Digital audio broadcasting

DBS



Direct broadcast
ing

satellite


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5


D/C



Down
-
converter

DLP



Digital light processing

DSL



Digital subscriber line

DTH



Direct
-
to
-
home

DV
B

H


Digital video broadcasting
-
handheld

DVB

RCS


Digital video broadcasting
-
return channel via satell
ite

DVB

S



Digital

video broadcasting
-
satellite

DVB

S2


Digital video broadcasting
-
satellite
-
second generation

DVB

SH


Digital video broadcasting
-
satellite services to handheld

DVB
-
T



Digital video broadcast
ing
-
terrestrial

DVB
-
T2


Digital video broadcast
ing
-
terrestrial
-
second generation

E
b
/N
0



Bit energy to noise spectral density ratio

E
s
/N
0



Symbol energy to noise spectral density ratio

E
-
S



Earth
-
to
-
space

ESPN



Entertainment and sports programming network

ETRI



Electronics and telecommunications research insti
tute

ETSI



European telecommunications standards institute

FCC



Federal
C
ommunications
C
ommission

FDD



Frequency
-
division duplex

FDM



Frequency
-
division multiplexing

FEC



Forward error correction

FES



Fixed earth station

FFT



Fast Fourier transform

FPGA



Field
-
programmable gate array

FSS



Fixed
-
satellite service

GEO



Geo
-
stationary orbit

GI



Guard interval

H
-
ARQ



Hybrid ARQ

HDTV



High definition television

HIHO



Hard
-
input hard
-
output

HPA



High
-
power amplifiers

HTS



High
-
throughput
satellites

IBO



Input
-
backoff

ICI



Inter
-
channel interference

IFFT



Inverse fast Fourier transform

6

Rep.

ITU
-
R S.2173


IPDC



Internet protocol datacast

IPoS



Internet protocol over satellite

ISI



Inter
-
symbol interference

LDPC



Low density parity check

LNA



Low noise
amplifier

LoS



Line
-
of
-
sight

LTE



Long term evolution

L
-
TWTA


Linearized travelling wave tube amplifier

LUT



Look up table

MAP



Maximum a posteriori

MC
-
CDMA


Multi
-
carrier code
-
division multiple access

MCSS



Multi
-
carrier satellite system

MEO



Medium
-
earth orbit

MF
-
TDMA


Multi
-
frequency TDMA

MLSD



Maximum likelihood sequence decoding

MODCOD


Modulation and channel code combination

MPU



Multi
-
carrier

processing unit

MSS



Mobile
-
satellite service

NACK



Negative acknowledgment message

O3B



Other 3 billion

OBO



Output
-
backoff

OBP



On
-
board processing

OECD



Organisation for economic co
-
operation and development

OFDM



Orthogonal frequency
-
division multiplexing

OFDMA


Orthogonal frequency
-
division multiplexing
-
frequency
-
division multiple

ac
cess

PAPR



Peak to average power ratio

PER



Packet error rate

PTS



Partial transmit sequence

QAM



Quadrature
a
mplitude
m
odulation


QEF



Quasi
-
error
-
free

QoS



Quality of service

QPSK



Quadrature phase
-
shift keying

RCFEC



Rate
-
compatible FEC

RF



Radio
-
frequency

RSC



Recursive systematic convolutional


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7


RSM



Regenerative satellite mesh

SCSS



Single
-
carrier satellite communications system

S
-
DARS


Satellite


digital audio radio service

SDR



Software defined radio

SDTV



Standard definition televis
ion

SFN



Single frequency network

SISO



Soft
-
input soft
-
output

SNR



Signal to noise ratio

SOVA



Soft output Viterbi algorithm

SR/ARQ


Selective
-
repeat ARQ

S
-
RAN



Satellite radio access network

SRS



Signal regeneration satellite

SSPA



Solid state
power amplifier

S

&

W/ARQ

Stop and wait ARQ

TC



Turbo codes

TD



Total degradation

TDD



Time
-
division duplex

TDM



Time
-
division multiplexing

TIA



Telecommunications industry association

TWTA



Travelling wave tube amplifier

U/C



Up
-
converter

UW



Unique word

VSA



V
ector signal analyser

VSG



V
ector signal generator

WLAN



Wireless local area network

1

Introduction

This Report presents an overview of multi
-
carrier based transmission techniqu
es over satellite links.
Sections

2 and

3 of this Report
give a general outline of satellite system design by providing
examples that include applications, scenarios and satellite services where multi
-
carrier
transmissions can be used.
In §

4 of this Report, basic digital satellite transmission models are
descri
bed, where more in
-
depth attention is given to each functional block and its operating
principles in §

5
-
8. Some recent satellite system standards are described in § 9, and simulation
results of multi
-
carrier transmissions over satellite links are evaluate
d in § 10 for various
performance parameters. Emerging trends in technology that may be used for satellite systems are
addressed in § 11.

8

Rep.

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-
R S.2173


2

Applications and scenarios

2.1

High definition television/Three
-
dimensional television

HDTV

High definition televisi
on (HDTV) is an example of a service that is traditionally delivered by
broadcast satellite services, such as direct
-
to
-
home (DTH). HDTV content is widely delivered by
satellite broadcast network using either the popular digital transmission standards: dig
ital video
broadcast
ing



satellite (DVB
-
S) or more recent digital video broadcast
ing



satellite


second
generation (DVB
-
S2)
1
. Currently, many terrestrial broadcasting networks are delivering HDTV
content using the digital video broadcast
ing



terrestrial (DVB
-
T) and digital video broadcast
ing



terrestrial


second generation (DVB
-
T2) standards, which are also digital formats. While in many
parts of the world, analogue broadcasting formats are still employed, the forced transition from
analo
gue to digital use of broadcast spectrum will eliminate its use in developed nations leading this
transition.

Satellite DTH presents a viable alternative to terrestrial wired and terrestrial broadcast for delivery
of HDTV, as well as, standard definition
television (SDTV) content, as it requires very little
infrastructure for the set
-
up of user terminals, while providing wide signal coverage.

The DVB
-
T and DVB
-
T2 formats make use of orthogonal frequency
-
division multiplexing
(OFDM) as a means of combating

the frequency selective fading that is prevalent in terrestrial
networks; but this technology has other benefits as well.
The use of OFDM allows for a single
frequency network (SFN)


where repeaters are used as gap fillers to enhance signal coverage


wi
thout the need for complex equalization filters. In addition, the use of OFDM for broadcast
systems allows for more efficient use of spectrum, since it reduces the number of guard bands
required between sub
-
channels. In § 5.2 the benefits of OFDM for satel
lites are discussed, which
includes the reduction in the number of guard bands required for satellite applications. However,
due to the high peak
-
to
-
average power ratio (PAPR) problems of OFDM for satellite high power
amplifiers and perhaps the generally s
low time
-
to
-
deployment of satellites, OFDM for broadcast
satellite has not caught on to date. Yet, it should be noted that new integrated MSS networks, which
are described in § 3.2, will make use of OFDM.

Spectrum requirements for HDTV broadcasts are typic
ally 3
-
5 times larger than those of SDTV,
depending on what type of video and audio compression are used [1].
As is explained in §

8.2,
the

use of DVB
-
S2 can increase the number of HDTV channels that a satellite DTH service
provider can distribute by rough
ly 33%.
This combined with new MPEG
-
4 compression can enable
the delivery of much more HDTV content in the programming of service providers.

3DTV

Three
-
dimensional television (3DTV) is a relatively new application that adds depth to a traditional
two
-
dimensional HDTV image. 3DTV
-
ready televisions have already been released to market, the
majority of which require the use of 3D glasses that produce a

stereoscopic effect to give depth to
the image.
Although, there are some “auto
-
stereoscopic” televisions that can produce 3DTV images
without the need for glasses. However, these televisions have a narrow viewing angle, lower
resolution and can cause eye
fatigue [2].
Television broadcasters have also begun to create 3D
content.
For example, in t
he United States of America
, the Entertainment and Sports Programming
Network (ESPN) offered limited 3D programs covering World Cup soccer games, while Sony and
Sky

Perfect JSAT Corp
oration

have broadcasted 3D World Cup programs in Japan.
Additionally,



1

For more information on DVB
-
S and DVB
-
S2 see § 8.


Rep.
ITU
-
R S.2173

9


the

Discovery Channel and ESPN are planning to broadcast dedicated 3DTV channels in the near
future. It is also worth mentioning that the DVB group is starting work on

their first phase to
standardize the 3DTV format [3].

There are still many questions to be sorted out for the 3DTV format, for which the answer will have
an impact on the bandwidth required to deliver its content. For example, the MPEG
-
2 and
H.264/MPEG
-
4
advanced video coding (AVC) codecs are currently both being used to deliver
SDTV and HDTV formats. Additionally, there are three standards for 3DTV signals, which affect
its delivery: checkerboard pattern, panels or full resolution. Checkerboard and panels

signals are
more simple signals that offer lower resolution than full resolution signals; however, they do not
require any additional bandwidth for signal delivery when compared with traditional HDTV.
Checkerboard pattern signals enjoy being
-
first to mark
et on digital light processing (DLP) devices;
although they are more difficult to compress than panels signals. The full resolution signal format is
created by adding a “depth signal” to the traditional HDTV signal. This additional signal causes
an

increas
e in the amount of bandwidth required to deliver the 3DTV signal; however, by using
MPEG’s latest multi
-
video coding compression standard for 3DTV, it is possible to compress the
full resolution signal such that an additional 70% of bandwidth is required w
hen compared with
an

HDTV signal [2]. This means that service providers could deliver roughly three 3DTV signals in
the same bandwidth as five HDTV signals. This could be more achievable if the operator were to
upgrade from DVB
-
S to DVB
-
S2, which makes use

of MPEG
-
4 and is more bandwidth efficient.
Otherwise, for a lower resolution, the service provider could implement simpler formats, making
the adoption of 3DTV by service providers a very cost
-
effective way to deliver new services at no
additional costs (
bandwidth).

2.2

Mobile multimedia

The mobile wireless industry has been enjoying very healthy growth over the past decade.
For

example, US
-
based AT &

T’s network has seen a 5

000% increase in data traffic on its mobile
network over the past 3 years, driven

in part by Apple’s iPhone.
Other US networks, such as
Verizon Wireless have also seen a substantial increase in data traffic on their networks [1].
The

increase in data traffic can be attributed to the widespread adoption of new “smartphones” and
other mo
bile internet devices (e.g. laptops with mobile internet cards), whose applications have ever
increasing demands for bandwidth. These devices are capable of accessing email, the internet and
using several types of applications that include access to severa
l forms of media (e.g. news, photos
and videos). As consumers expect their devices to be connected “wherever and whenever”,
the

demand for ubiquitous mobile devices will become more prevalent.

Terrestrial connections outside of city centres can be limited

and as a result, the use of satellites for
mobile broadband and multimedia is an interesting complement or alternative [4]. The use of
ubiquitous hybrid satellite/terrestrial handhelds as a solution for the delivery of mobile multimedia
is becoming a prev
alent idea. The digital video broadcast


satellite handheld (DVB
-
SH) standard
specifies the transmission scheme for a hybrid satellite/terrestrial device that can receive digital
broadcasts for mobile television from both terrestrial and satellite sources
. Another way to leverage
satellites for mobile multimedia is the use of integrated mobile
-
satellite service (MSS) systems,
which are described in § 3.2. A mobile smartphone could be switched between the satellite
component and terrestrial component of the

network depending on coverage. In this way, some or
all
2

applications used to access content on the terrestrial network could be accessed using the
satellite component of the integrated network.
An example of a smartphone developed for use on



2

Some applications may be latency sensitive, which may be problematic for geosynchronous orbit satellite
networks, where too much latency may render certain applications unusable.

10

Rep.

ITU
-
R S.2173


an

integrate
d MSS is the Genus™ smartphone released by Terrestar, that will make use of
AT

&

T’s network (for the terrestrial component) and Terrestar’s satellite network (for the satellite
component).
It is also worth noting that the digital video broadcast


return
channel via satellite
(DVB
-
RCS) standard has a mobile option specified in it. This option would allow for mobile and
nomadic interactive applications


which can include broadband multimedia access


over satellite.

2.3

Broadband Internet

The definition of

broadband differs depending on which government; committee; agency;
association; or body is discussing it. In general, broadband Internet is defined as high data
rate/speed Internet access that is always connected
3

at speeds much faster than traditional d
ial
-
up
Internet (56 kbit/s). Modern definitions for broadband can range from 5 to 2

000 times the rate of
56

kbit/s dial
-
up Internet [5]. However, the definition of broadband Internet will continue to evolve
as data rates continue to grow. In [6], the Orga
nisation for Economic Co
-
operation and
Development (OECD), defines broadband as Internet connection at rates exceeding 256 kbit/s for
downloads. In [7], the FCC defines the broadband in seven tiers of service, with the first tier (“basic
broadband”) having

a rate in the range of 768
-
1

500 kbit/s for downloads. In [8], Industry Canada


the department of the government of Canada managing the country’s broadband plan


defines
broadband as having a connection of at least 1.5 Mbit/s download and 384 kbit/s upl
oad speeds.
In

[5], the ITU defines broadband Internet as an Internet service providing data rates of 1.5
-
2 Mbit/s
or more for download, which is the median download speed for a typical digital subscriber line
(DSL) connection.

National broadband strategie
s, which seek nationwide connectivity through government
subsidization with the goal of further economic growth, are leading to increased interest in the
deployment of broadband Internet. Many of these strategies seek to connect citizens that are yet to
be

served by broadband, with each institution having their own particular definition of what
constitutes a broadband connection. These citizens are typically in rural communities where
broadband penetration is low due to the high cost of building infrastruct
ure.
As acknowledged in the
federal communications commission’s (FCC) National Broadband Plan [9], broadband satellite
could be a viable way to achieve broadband penetration for those in communities that are
underserved and where it would be prohibitively
expensive to build terrestrial infrastructure [10].
Currently, most satellite Internet services are provided at speeds that are lower than the minimum
data rates set forth in [5, 7, 8], however, there are many that would qualify as a broadband service
unde
r the OECD figures. To access government subsidies for Internet broadband, satellite
companies will have to meet the minimum rates as defined in each national strategy.

High
-
throughput satellites (HTS) are being developed by several companies worldwide th
at could
meet the minimum rates defined for broadband. For example, in the US, Viasat and Hughes
Network Systems are developing HTS, where they are expected to provide download speeds of
2
-
10 Mbit/s and 5
-
25 Mbit/s, respectively [9]. However, these HTS wil
l operate in geosynchronous
orbits and, as a result, are more susceptible to latency than LEO and MEO satellite. Too much
latency could hinder the use of interactive real
-
time applications. However, the latency issue is less
relevant for applications that
require best
-
effort network performance, such as e
-
mail and Internet
browsing. To attempt to overcome the latency issue, the Other 3 Billion (O3B) [11] partnership is
designing a satellite broadband Internet service that will use medium
-
earth orbit (MEO) s
atellites.
This design will include the deployment of several MEO satellites


at 1/5 the distance of GEO
satellites


that use flexible spot beams to connect developing nations to broadband Internet. O3B



3

Always connected means that the service is available without havi
ng to fir
st connect to it, such
as is the
case with dial
-
up.


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11


claims that by reducing the distance of satellites
from the earth, the latency of round
-
trip delay will
be far less.

Although standards that define user interaction via satellite, such as DVB
-
RCS, do not include the
use of OFDM, its use may be of some benefit.
Although not a necessity, routing and capacity

flexibility are desirable characteristics for broadband satellites; where efficient routing and capacity
allocation greatly improves the performance of the system. More flexible routing and capacity can
be achieved by using multiple carriers per beam in a

frequency
-
division multiplexing (FDM)
manner. That is, by implementing OFDM. This could be easily accomplished where on
-
board
processing is available on the satellite by using efficient fast
-
Fourier transform (FFT) engines.

3

Satellite systems examples

3.
1

17/24 and 21/24 GHz BSS systems

A broadcasting
-
satellite service (BSS) is defined as a radiocommunication service in which signals
transmitted or re
-
transmitted by space stations are intended for direct reception by the general
public.
The satellites imp
lemented for BSS are often called direct broadcast
ing
-
satellites (DBS
’s
).
The

antennas required for BSSs should be smaller than those used for fixed
-
satellite services
(FSS
’s
) [12]. Figure 1 demonstrates an example of a BSS with a star
-
network topology, where
communication is one
-
way, from feeder link to user terminals via satellite.

In 2003, new frequency allocations were made for BSS, which vary dependin
g upon the Region of

operation.

FIGURE 1

Example of BSS


12

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-
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In Region 2, 17.3
-
17.8 GHz for BSS and 24.75
-
25.25 GHz for FSS feeder links for BSS are
allocated for operation as a primary service.
These bands are shared with the Earth
-
to
-
space (E
-
S)
segment of FSS
’s
. Where in the 17.3
-
17.8 GHz bands the use of E
-
S segments for FSS is strictly
limited to feeder links for BSS. It should also be noted that the 24.75
-
25.25 GHz bands are shared
with FSS
’s
; however, feeder link for BSSs have priority over o
ther types of FSS
’s
.

In Regions 1 and 3, the 21.4
-
22.0 GHz band BSS can operate as a primary service. Region 3 also
uses 24.75
-
25.25 GHz for FSS feederlinks for BSS, where FSS feederlinks for BSS have priority
over other types of FSSs.
In particular, the i
ntroduction of HDTV BSSs in the 21.4
-
22 GHz band is
governed by Resolution 525 (Rev.WRC
-
07) of the Radio Regulations (RR). In its Annex (Section I


General Provisions) Resolution 525 (Rev.WRC
-
07) states that all services other than the BSS
,

in
the band 21
.4
-
22.0 GHz in Regions 1 and 3 operating in accordance with the Table of Frequency
allocations may operate subject to not causing harmful interference to BSS (HDTV) systems nor
claiming protection from such systems. It further elaborates that it shall be u
nderstood that the
introduction of an operational BSS (HDTV) system in the band 21.4
-
22.0 GHz in Regions 1 and 3
should be regulated by an interim procedure in a

flexible and equitable manner until the date to be
decided by WRC
-
12.

3.2

Integrated MSS syst
ems

An integrated MSS system is defined in [13, 14] as a system employing a satellite component and
ground component where the g
round component is complementary to the satellite component and
operates as and is an integral part of the MSS system. In such s
ystems the ground component is
controlled by the satellite resource and network management system. Further, the ground
component uses the same portions of MSS frequency bands as the associated operational MSS.

An integrated system provides a combined (inte
grated) single network that uses both a traditional
MSS link and terrestrial transmission paths to serve mobile end
-
users.
A typical integrated system
comprises one or more multi
-
spot beam satellites and a nation
-
wide or regional ensemble of
terrestrial ce
ll sites, where both terrestrial and space components communicate with mobile
terminals using a common set of MSS frequencies.
Such systems are referred to as MSS
-
ancillary
terrestrial component (MSS
-
ATC) in the United States of America and Canada, and
MSS
-
complementary ground component (MSS
-
CGC) in Europe and are implemented in the
1
-
3

GHz bands.

Integrated systems will likely have various service components, including traditional MSS services.
A rather large portion of this portfolio of services will be devoted to the provision of broadband
services


including multimedia broadband services


to h
andheld or portable terminals. These
handhelds and portable terminals are expected to have form and cost factors very similar to
terrestrial cellular terminals of today [15]. There is a variety of approaches that system operators
may choose to implement th
eir baseline service provisioning, coverage goals and end
-
user demand.

Initial examples of integrated satellite systems are represented by satellite digital audio broadcasting
(DAB) to mobile receivers mounted on vehicles in the early 2000s. Broadcasting of high
-
quality
digital radio channels to mobile users was achieved by m
eans of S
-
band frequencies around 2 GHz.
This was called Satellite


Digital Audio Radio Service (S
-
DARS).
Two companies in the USA,
XM
-
Radio and Sirius, provided these services [16, 17]. The remarkable success achieved by
XM
-
Radio and Sirius was based on
traditional counter
-
measures such as a high link margin;
time,

frequency and satellite diversity; and the use of terrestrial repeaters in urban areas.

More recently, in Korea and Japan, S
-
DMB service to hand
-
held user terminals was successfully
deployed ut
ilizing a geostationary (GEO) satellite [18]. The S
-
DMB system was based on code
division multiplexing (CDM) technology, described in Recommendation ITU
-
R BO.1130
-
4 [19].
In

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13


Europe, the

Unlimited Mobile TV


concept was introduced [20], which is based on t
he ETSI
standard of DVB


Satellite services to Handheld (SH) devices published in March 2008 [21].
With

this concept, a hand
-
held mobile terminal is supposed to receive broadcast signals from both
the satellite and the CGCs.
Video services from ICO G1 (a
GEO satellite that covers the entire
United States of America, Puerto Rico and the US Virgin Islands) are based on the DVB
-
SH
standard as well.
The DVB
-
SH system was designed for frequencies below 3 GHz (S
-
band). It
complements the existing DVB
-
handheld (D
VB
-
H) terrestrial coverage and analogously uses the
DVB IP datacast (IPDC) set for content delivery, electronic service guide and service purchase, and
protection standards.

3.3

Ka
-
b
and broadband systems

Increasing demand for broadband services can be eff
ectively handled by a satellite system using
high frequency bands such as
the Ku and Ka bands. Especially, to provide high
-
speed Internet and
television (TV) services to maritime and air vehicles, a satellite system may be the only possible
option. In this

case, an active array antenna that is mounted on a moving vehicle is used to track
a

satellite and provide seamless connections.

For Ka
-
band broadband systems, the volume of traffic on the forward link, which provides
connections from the satellite gatew
ay to the user terminals, is much greater than that on the return
link, which provides connections from the user terminals to the satellite gateway. Recommendation
ITU
-
R BO.1709
-
1 specifies three air interface standards, which can be used to implement
broa
dband satellite networks [22], as shown in Table

1.

TABLE 1

Air interface standards for broadband satellite systems in

Recommendation ITU
-
R S.1709
-
1

Standard name

Scheme

ETSI EN 301 790

TIA
-
1008
-
A

ETSI
-
RSM
-
A

Network topology

Star or mesh

Star

Star or
mesh

Forward link sch
eme

DVB
-
S

DVB
-
S

High rate TDMA

Forward link data rate
(Mbit/s)

1
-
45

1
-
45

100, 133.33, 400

Return link modulation

QPSK

O
-
QPSK

O
-
QPSK

Return link multiple access

Multi
-
frequency
(MF)
-
TDMA

MF
-
TDMA

FDMA
-
TDMA

Return link data rate

No

restriction

64, 128, 256, 512, 1

024,
2

048 ksymbol/s

2, 16, 128, 512 kbits/s


The DVB via satellite (DVB
-
S) standard, which is specified as the forward link scheme for the first
and second air interfaces in Table 1, describes an air interface for the satellite multi
-
program TV
service.
As a result of the rapid evolution of digital s
atellite communication technology since the
introduction of DVB
-
S
4

in 1994, a second generation standard for digital video broadcasting via
satellite, DVB
-
S2, has been published. Remarkable improvements are achievable for DVB
-
S2,
including a new channel co
ding scheme and higher
-
order modulation, which used as part of



4

For more information on DVB
-
S see § 9.1.

14

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3


an

adaptive coding and modulation (ACM) technique. ACM is used mainly to compensate for rain
attenuation, which is more prevalent in high frequency bands. The new channel code in DVB
-
S2 is
a co
ncatenated code with low density parity check (LDPC) codes and Bose

Chaudhuri

Hocquenghem (BCH) codes. The higher
-
order modulation scheme is based on amplitude and phase
shift keying (APSK) constellations up to order 32, which has been shown to be more rob
ust to
non
-
linearities caused by high power amplifier distortion than quadrature amplitude modulation
(QAM) [23]
5
.

The air interface in the first column of Table 1 is called a DVB
-

return channel by satellite
(DVB
-
RCS), and in combination with DVB
-
S/S2 it
provides two
-
way broadband satellite systems.
Because the current version of DVB
-
RCS does not consider mobile service environments,
an

advanced version for comprehensive support of mobile and nomadic terminals is currently under
study
6
.
The second air inte
rface in Table 1 is the Internet Protocol over Satellite (IPoS) standard that
has been developed by Telecommunications Industry Association (TIA) in the USA. The third air
interface in Table 1 is the Broadband Satellite Multimedia (BSM) standard developed
by ETSI.
Important characteristics of the BSM architecture are that its operational functions are separated
into two types: satellite dependent functions and satellite independent functions. The

purpose of this
separation is to provide the capacity to inco
rporate future market developments, as well as the
flexibility to include different market segment
-
based solutions in the higher layers of the protocol
stack. Among the several types of BSM air interface families, Regenerative Satellite Mesh (RSM)


A is b
ased on a satellite with onboard processing (OBP) such as SPACEWAY by Hughes Network
Systems, which supports a fully meshed topology. With RSM
-
A, data can be transmitted between
any pair of satellite terminals in a single hop.

The Electronics and Telecommu
nication Research Institute (ETRI) in Korea has developed
a

mobile broadband interactive satellite access technology (MoBISAT) system based on the
DVB
-
S/DVB
-
RCS standard for satellite video multicasting and Internet via wireless local area
networks (WLANs)

[24].
The system can provide broadband services to passengers and the crews
of land, maritime, and air vehicles, via installation of group user terminals with a two
-
way active
antenna. To access the Internet inside vehicles, WLANs can be provided, enablin
g the use of laptop
PCs or PDAs on board the vehicle.

4

System implementation methods

4.1

Single and multi
-
beam satellite systems

A geosynchronous orbit satellite communication system with large satellite antenna(s) can provide
high data rate (or
high spee
d) services to small user terminals thanks to a large satellite antenna
gain. A
geosynchronous orbit

satellite system with multi
-
beam antennas has a larger capacity than a
system with a single global beam over the same service area [25]. For a multi
-
beam
g
eosynchronous orbit

satellite system, we can easily synchronize all the downlink (satellite
-
to
-
user
link) signals from a single satellite because the satellite is the only source of the signal. For a
synchronous multi
-
beam satellite system, the beam signal
s received by the user are also
synchronized, regardless of the user location. Therefore, inter
-
beam interference can be mitigated.




5

For more
information

on DVB
-
S2 see § 9.2.

6

For more
information

on D
VB
-
RCS see § 8.3.


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15


Figure 2 illustrates a multi
-
beam satellite system providing IP packet services.
Services for mobile
users are linked to a
terrestrial IP core network through a fixed earth station (FES) and satellite. The

geosynchronous orbit

satellite has to be equipped with a large directional antenna, in order of about
20 meters, so as to be able to provide high
-
speed services for nomadic
and portable terminals
equipped with small antennas. The FES performs adaptive resource allocation on the satellite
downlink and is a

gateway that links user services to the terrestrial network. When the satellite has
on
-
board processing capability, it can

perform adaptive resource allocation [26].

FIGURE 2

Multi
-
beam satellite system providing IP packet services


4.2

Digital satellite transmission system

Figure
3

shows a simplified digital satellite transmission system viewed for a single link.
Depending on how the system is
implemented, a number of modulated and filtered data can be
multiplexed and sent through this link. In this section, the basic principles of
each functional block
are described and a detailed description of each follows in
§

5
-
7.

The source encoder in Fig.
3

transforms information into a given digital representation, and can
possibly include a mechanism for the removal of any inherent redundan
cy within the data.
Channel
encoding is applied to the output of the source encoder. Channel coding, which is also called error
control coding, is a digital processing technique aimed at averaging the effects of channel noise
over several transmitted signa
ls. A detailed discussion on channel coding is provided in
§

7.

The modulator converts the digital information into a signal that can be easily transmitted through
the channel. This is usually achieved by translating the frequency of the source informatio
n onto
a

carrier frequency, which is a frequency that is much higher than that of the un
-
modulated source
signal. To modulate the digital information, the digital stream is first put into a baseband
representation. The most commonly used digital modulation

scheme for satellite systems is phase
16

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shift keying (PSK), in which the data determines the phase of the transmitted signal. In bandwidth
-
limited conditions


as is common for satellite systems


utilization of multi
-
level modulation most
often will increa
se the bit
-
rate of the satellite system. Discussion on multi
-
level modulation schemes
specified in recent satellite system standards are described in
§

9.


FIGURE
3

Digital satellite transmission system



This Report emphas
izes the utilization of a multi
-
carrier modulation scheme and its utilization for
multiple access. Section

5 describes details on multi
-
carrier modulations.

After filtering is applied to remove the sideband signals, the modulated signal is amplified by a h
igh
power amplifier (HPA).
As the transmitted signal travels a long distance, and as a result, is subject
to larger propagation loss than typical terrestrial systems, a very high power amplification is
required. Generally two different types of the HPA

s a
re used by satellite systems: travelling wave
tube amplifier (TWTA) and solid state power amplifier (SSPA).

At the receiver, signal recovery operations are applied in the reverse order that they were carried out
by the transmitter.
The received signal is
passed through a low noise amplifier (LNA) and then
filtered to remove the sideband signals. The demodulator uses the filtered signal to create decisions
on the transmitted digital information and passes those decisions on to channel decoder. Depending
on
the requirements of the channel decoder, the decisions produced by the demodulator can be hard
(i.e. zero or one), or soft. Soft
-
decision decoding uses metrics based on the likelihood that
a

transmitted bit is zero or one to make a final decision. This sof
t
-
decision metric is required by
iterative channel decoders, which are discussed in detail in
§

7.


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5

Multi
-
carrier and multiple access systems

5.1

Basics of multi
-
carrier transmission

Multi
-
carrier techniques have become very popular for high data rate
applications, and many
next
-
generation terrestrial standards include multi
-
carrier
-
based modulation and multiple access
techniques.
The most powerful advantage of multi
-
carrier techniques is their capability to
eliminating problem deriving from intersymbol

interference (ISI), alleviating the high burden of
complex time
-
domain equalizers while maintaining a high rate of transmission. Particularly, in
rapidly varying channels, such as wireless channels, the burden of maintaining accurate values for a
many
-
tap

equalizer can be a major burden.

In order to have a channel that is ISI
-
free, the symbol time
Ts

has to be much longer than the delay
spread of the channel,

. Typically, it is assumed that
Ts

should be at least ten times larger than


in
order to satisfy

an ISI
-
free condition. The bit error rate (BER) performance of a digital
communication system seriously degrades if
Ts

approaches

. In order to satisfy high data rate
requirements, the desired Ts is usually much smaller than

, which leads to severe ISI
problems.
In

the frequency domain, the channel characteristics become selective. In this situation, channel
gains are attenuated and enhanced over the system bandwidth,
Bs
, because the coherence bandwidth
of the channel,
Bc

is larger than
Bs
.

Figure 4 ill
ustrates the basic concept of multi
-
carrier transmissions. Multi
-
carrier modulation
divides the high
-
rate transmit bit
-
stream into
N

lower
-
rate substreams, each of which has
NTs





,
and thus ISI problem can effectively be eliminated.
These individual sub
streams can then be sent
over
N

parallel subchannels, maintaining the total desired data rate. In the frequency domain,
the

subcarriers have
N

times less bandwidth, which equivalently ensures a much smaller subcarrier
bandwidth than
Bc
; or in other words,
a relatively flat fading condition for each substream.

Orthogonal frequency division multiplexing (OFDM) is a technology where each subchannel in
a

multi
-
carrier transmission is orthogonal to all other subchannels.
By using OFDM, the bandwidth
efficiency
can be further increased when compared with a conventional frequency division
multiplexing (FDM) system. In addition, no complex carrier de
-
multiplexing filters are needed to
separate the carriers, as simple fast Fourier transform (FFT) engines can be used

to do
demodulation.
Figure 5 shows a simple block diagram for an OFDM modulator.
N

baseband
modulated symbols are first converted to
N

parallel streams with symbol periods that are
N

times
longer, and are then passed through an inverse FFT (IFFT) engine.
Although
N

times longer, the
lengthened symbol duration can reduce the problems arising from ISI between the modulated
symbols that compose a particular OFDM symbol.
If a delay spread remains between OFDM
symbols, a guard time or guard interval (GI) is ins
erted between OFDM symbols in order to
eliminate the remaining delay spread. As plain insertion of a GI may cause inter channel
interference (ICI) between sub
-
carriers, a cyclically extended GI must be inserted. This cyclic GI is
called a cyclic prefix (CP
), as each OFDM symbol is preceded by a periodic extension of the signal
itself.

18

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-
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FIGURE
4

Basic concept of multi
-
carrier transmission



FIGURE
5

Block diagram of an OFDM modulator



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19


5.2

Multi
-
carrier transmission over a
satellite link

Many terrestrial system standards have adopted OFDM
-
based transmission schemes. Considering
that OFDM
-
type systems are largely used in terrestrial networks as a means of providing good
spectral and energy efficiency over frequency selective
channels, the idea of utilizing them for
satellite systems may seem to lack justification. This is due to the fact that conventional satellite
systems do not experience frequency selective channel. Moreover, multi
-
carrier systems suffer from
high peak to a
verage power ratio (PAPR) problems, which impose a high burden on the high power
amplifiers of satellite systems.

Even with the aforementioned obstacles, there are a number of factors that may make this
technology attractive for satellite services [27]:

1
.

A satellite component of a next
-
generation network may be regarded as providing coverage
extension for service continuity of the terrestrial component by allowing for vertical
handover. For cost
-
effective vertical handover, future satellite radio interf
aces may be
compatible and have a high degree of commonality with terrestrial interfaces. This may
enable the reuse of terrestrial component technology, to minimize the user terminal chipset
and network equipment for low cost and fast development.

2
.

In broadband satellite communications where routing and capacity flexibility over the
coverage area require the use of multiple carriers per beam with FDM.

3
.

In a scenario where an on
-
board switch is also used to route the time division multiplexing
(TDM
) data from a given gateway to different beams, OFDM may simplify the architecture
of the on
-
board switch. No complex carrier de
-
multiplexing filters are needed to separate
the carriers because simple FFT engines can be used.

4
.

Most importantly, OFDM tec
hniques may represent a means to increase the spectral
efficiency by compacting carriers, eliminating guard
-
bands for applications requiring high
speed transmissions.

An existing example where OFDM transmission is adopted for satellite is the DVB
-
SH stand
ard.
DVB
-
SH uses the same signal format defined in DVB
-
H for terrestrial systems. The main reason
for the adoption of OFDM in DVB
-
SH is that satellite and terrestrial transmitters in this network
topology form a single frequency network (SFN). This kind of

configuration uses the available
frequency resources efficiently, as the same frequencies are used for both satellite and terrestrial
transmitters because the same content is transmitted on both links.
Terrestrial transmitters can be
fed by receiving the
satellite signal, if the transmitted and received signal is sufficiently isolated
[28]. In addition to the high spectral efficiency of multi
-
carrier transmission, there is the added
flexibility of allocating resources for adaptive use, i.e. not only in ter
ms of time but also in terms of
frequency domains [26].
In order to reduce the high PAPR incurred in the multi
-
carrier transmission
technique, a number of approaches can be applied, which are discussed in § 5.

5.3

Multi
-
carrier based multiple access schem
es

Due to increasing interests in multi
-
carrier transmissions schemes, the multiple access schemes
associated with them are considered as one of the promising technologies for next
-
generation
networks. Examples of these are multi
-
carrier code division mult
iple
-
access (MC
-
CDMA),
multi
-
frequency time division multiple access (MF
-
TDMA) and orthogonal frequency division
multiplexing


or frequency division multiple access (OFDM
-
FDMA; OFDMA). An example
application where one of these schemes is employed is MF
-
TD
MA, as defined in DVB
-
RCS.

20

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-
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The basic concept of multi
-
carrier based multiple access scheme is that all N subcarriers are shared
by multiple users in the system.
Figure 6 compares the basic concept of OFDMA, MF
-
TDMA, and
MC
-
CDMA.
In OFDMA, a number of user
s share subcarriers, of which a portion is allocated to
each user. On the other hand, in MC
-
CDMA, each user’s data is first spread by an allocated
spreading code and are then transmitted using all the subcarriers simultaneously.

FIGURE
6

Comparison of OFDM
A, MF
-
TDMA and MC
-
CDMA


6

P
eak
-
to
-
average power ratio reduction technologies

6.1

Introduction

One of the major drawbacks of multi
-
carrier schemes is the high peak
-
to
-
average power ratio
(PAPR) of the transmitted signal. In s
atellite communication systems, the earth station transmitter
and the satellite transponder employ high
-
power amplifiers (HPA)s. It is desirable to operate the
HPAs as close as possible to their saturating point to maximize their radio
-
frequency (RF) power

efficiency. However, operation of these HPAs near their saturation point seriously degrades the
performance of modulation schemes which do not have a constant envelope. Therefore,
the

selection of a suitable modulation scheme combined with a PAPR reductio
n method is required
in order to reap the advantages of multi
-
carrier satellite systems. It should be noted that non
-
discrete
individual beam signals described in the MC
-
CDMA technique are often referred to as OFDM
signals. The following section reviews a
number of techniques that may be used to reduce the
PAPR of OFDM
-
type signals.


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21


6.2

P
eak
-
to
-
average power ratio reduction techniques

Block scaling

In the case of block scaling, critical OFDM symbols are identified by comparing their amplitude
with one or more thresholds. If any of the samples in a symbol exceed the thresholds, then the
amplitude of the whole symbol is reduced by multiplying each of i
ts samples by a factor less than
unity. This method requires the transmission of side information to tell the receiver how to restore
the signal to its original amplitude. Although this causes noise enhancement, it is expected that
coding can more easily c
orrect errors introduced by Gaussian noise than bursts of errors resulting
from the clipping effect of a saturated amplifier.

Channel coding

A number of techniques suggest employing a proper form of channel coding to avoid transmission
of the symbols that

exhibit a high PAPR. The basic idea of these techniques is to exploit the
redundancy introduced by a properly chosen code, which allows the system to refrain from
transmitting those sequences that give rise to a high PAPR. It is also desirable to exploit
the
properties of the code to perform some sort of error correction.

By considering the instantaneous power of an OFDM signal in its discrete time form, it can be
shown that the resulting PAPR depends on the auto
-
correlation function of the sequence of dat
a to
be transmitted once modulation parameters are fixed. Studies have demonstrated that it would be
possible to transmit only the symbols belonging to a subset which would ensure a lower PAPR.
However, the computational requirements to identify a set of s
uitable sequences are prohibitively
large for practical numbers of sub
-
carriers. An automatic coding technique is preferred for
an

efficient implementation.

Golay sequences are known to have the interesting property of producing a PAPR which is always
limi
ted, when transmitted with phase
-
modulated carriers. The PAPR for a Golay sequence is upper
bound at 3 dB. However, the codes have a rate ranging from 0.3 to 0.4. It has been demonstrated
that as the number of sub
-
carriers increases, the redundancy require
d for proper PAPR reduction is
high, for only a modest error correction capability. This substantially reduces the spectral efficiency
of the system.

Partial transmit sequence scheme

Partial transmit sequence (PTS) scheme is one of the most efficient appro
aches in terms of PAPR
reduction performance. With this scheme, a phase shift is applied to disjoint sub
-
blocks of the data
sequence, hence the name partial transmit sequence. The combination of such blocks with different
phase shifts gives the wanted diff
erent alternatives for transmission. PTS is flexible in that it can be
used with an arbitrary number of subcarriers and any type of modulation scheme. However, it is
difficult to implement because its implementation has exponentially increasing complexity;

although, a reduced
-
complexity technique can be used at the sacrifice of some performance.
The

PTS scheme also requires the transmission of a substantial amount of side information.
Reference [29] investigated a reduced complexity PTS scheme for a satelli
te system using
MC
-
CDMA.

Sub
-
carrier scrambling

For subcarrier scrambling, each user’s data symbol is spread by the orthogonal user code in the
frequency domain and thus, there is a correlated pattern among subcarriers. Since the correlated
pattern among s
ubcarriers influences the PAPR, the PAPR is highly dependent on the patterns of
the orthogonal user codes used. This implies that the PAPR of the multi
-
carrier signal will be
reduced if disturbances are introduced into the correlation or coherence among th
e subcarriers [30].

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One simple way to accomplish this is to scramble the polarities of the subcarriers. Randomly
scrambling the polarities of the subcarriers removes the correlation, and limits the coherent
combination among the subcarriers irrespective of

the user codes used. Therefore, the PAPR of each
user code (or code combinations) becomes statistically equivalent to the others and is much smaller
than that of the worst
-
case PAPR of no scrambling.

A further PAPR reduction can be achieved by using the f
ixed scrambling patterns that are
separately optimized for each user code (or user code combination) and holding the scrambling
pattern while the same code combination is maintained. If the optimum fixed scrambling patterns
for each user code combination a
re searched prior to system operation, they can be allocated
dynamically according to the current user code combination during the system operation. Besides
its great PAPR reduction capability, this method has two advantages. First, it does not require any

additional real
-
time computation for PAPR reduction. Secondly, it requires negligible side
information. In this scheme, the system has to broadcast to the users which scramble pattern is
currently in use. The amount of information for this is relatively s
mall and needs to be sent only at
the beginning of each connection/disconnection procedure.

Dynamic allocation of the scrambling pattern may add a processing load in busy traffic conditions.
In order to avoid this, the system can employ a sub
-
optimum solut
ion. From an extensive search of
scrambling patterns, a single scrambling pattern that maintains a PAPR close to the minimum over
all code combinations can be found. This sub
-
optimum solution can be found before system
operation via extensive computer sear
ch, meaning that the system can reduce the PAPR without any
additional processing burden or any side information.

Tone reservation

In this scheme, the system abstains from transmitting data on a limited number of sub
-
carriers,
which are then modulated in a

way that opposes the high peaks that result from data modulated
sub
-
carriers. With a proper combination of the reserved tones, a peak
-
reduction signal can be
superposed onto the information bearing OFDM symbol, such that the peak
-
reduction signal is in
ph
ase opposition to the original symbol at the time instants where there are peak amplitudes.
Consequently, the modified symbol has a lower PAPR.

The generation of the peak
-
reduction signal needs to take place for each OFDM symbol through an

optimization al
gorithm.
The number of sub
-
carriers allocated to peak
-
reduction must be sufficient
to be effective, but sufficiently small so as not to excessively sacrifice the information rate.

Pulse superposition

The pulse superposition technique acts directly on the h
ighest power peaks of the OFDM signal.
Once the signal has been generated, an anti
-
peak signal is added to it, resulting in a signal having
a

lower PAPR. The peak reduction is thus completely executed in the time domain, and

can use all
available informati
on on the characteristics of the signal to be modified because the signal has
already been generated when the PAPR reduction is applied. Pulse superposition obtains a reduced
PAPR by construction. The generation of the anti
-
peak signal is made directly in
the time domain
and thus does not necessitate additional inverse fast Fourier transform (IFFT) processing.

The anti
-
peak signal is built from the replica of a carefully selected elementary pulse. The shape of
the elementary pulse is very important, because

it is expected to reduce a high peak without leading
to the creation of secondary peaks, and it must have a spectrum without components that are out of
the band of the original signal. The effectiveness of the pulse superposition technique is similar to
t
hat of the block scaling approach, but it does not suffer from noise enhancement because weak
signals are unaffected.


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23


Non
-
linear companding transforms

Non
-
linear companding transforms compand the original OFDM signals using strict monotone
increasing funct
ions. The companded signals at the transmitter can be recovered correctly through
the corresponding inversion of the non
-
linear transform function at the receiver. Non
-
linear
companding transforms enlarge the small signals and compress the large signals to

increase the
immunity of small signals to noise.

The non
-
linear companding transform is in essence a type of clipping scheme, but it does not suffer
from the in
-
band distortion, out
-
of
-
band radiation and peak regrowth after digital to analogue
conversion
that affects simple clipping schemes.

Since the distribution of the original OFDM
signals are known to have Rayleigh distributions, the non
-
linear companding transform function can
be obtained theoretically on the basis of the desired distribution of the c
ompanded OFDM signals.
Two types of non
-
linear companding transforms based on the error function and the exponential
function have been proposed.

Non
-
linear companding transforms are a type of non
-
linear process that may lead to significant
distortion and
performance loss caused by companding noise. However, unlike AWGN,
companding noise is generated by a known process that can be recreated at the receiver,
and

subsequently removed. An iterative type receiver has been proposed to eliminate companding
noise
for a companded and filtered OFDM system.

6.3

CI
-
OFDM

6.3.1

Introduction

The implementation of a multi
-
carrier satellite system is impeded by its HPA’s inability to deal with
the high PAPR of OFDM signals. This makes PAPR suppression techniques highly attr
active for
the design of a multi
-
carrier satellite system.
Carrier Interferometry OFDM (CI
-
OFDM) [3
1
] is a

low
-
complexity PAPR suppression scheme that has been shown to mitigate the PAPR of a

communications system, while providing good packet
-
error rate (P
ER) performance


as is shown
in § 10.2.2 of this Report.

6.3.2

CI
-
spreading technology

CI
-
OFDM is a type of sub
-
carrier scrambling technology that can be implemented in an OFDM
system at the cost of an additional FFT module at the transmitter and receive

end. Therefore,
the

number of OFDM car
ri
ers has little impact in increasing the scale of hardware [3
2
].

Figure
7
shows the basic configuration of a CI
-
OFDM transmitter, as described in [3
1
]. Here,
modulated symbol samples,
s
i
, are multiplied by a spreadin
g code
CI
i
, where
CI
i
k

= exp(j2
π
ki
/
N
)
,
and multiplied by each subcarrier.
This effectively creates a CI
-
OFDM signal where each symbol is
transmitted on all subcarriers. The CI
-
OFDM

modulated

samples,
x
i
:





(1)


are then combined to create a multi
-
carrier vector,
x
, representing the time samples of the
CI
-
OFDM modulated signal. A GI, which is chosen based on the characteristics of the channel,
is

appended to
x

to avoid interference between subsequent multi
-
carrier

symbols due to the delay
spread of the channel. Each individually spread OFDM symbol has its peak when all other symbols
are at their minimum, avoiding the constructive addition of subcarrier peaks that leads to high
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a

PAPR. This creates a nearly flat CI
-
OFDM signal, as demonstrated in Fig
.

8
. It should be noted
that the summations in
equation
(1) are two inverse discrete Fourier transforms (IDFT)s of
s
i
. The
two IDFT operations performed in
equation
(1) can be replaced by two inverse FFT engines [32], as
depicted in Fig
.

9
.

FIGURE
7

Basic configuration of CI
-
OFDM transmitter


FIGURE
8

Peak reduction effect of CI
-
OFDM



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FIGURE
9

Configuration of CI
-
OFDM transmitter using FFT engines


6.4

Power amplifier linearization: a technique to reduce the effect of PAPR

An HPA’s non
-
linear behaviour is one of the most significant factors in determining the PAPR
performance, and the linearization of the power
amplifier is the most effective method to be used in
overcoming this difficulty. Linearization of the power amplifier is more important to multi
-
carrier
systems, because they are more sensitive to non
-
linear distortions due to their comparatively high
PAPR
. Over the years, a number of linearization technologies have been developed and
predistortion has been the most common approach utilised by new systems today. Although
predistortion techniques are not a direct way of reducing PAPR, they can be an effectiv
e solution to
reduce the performance degradation due to PAPR.

The essence of predistortion is to precede an HPA with a non
-
linearity that is the inverse of the
HPA’s non
-
linear behaviour. This non
-
linear device can be designed digitally using a mapping
pre
distorter (a look
-
up table: LUT) or a polynomial function based on Cartesian or polar
representation. All implementations of predistortion should be adaptive to changes in operating
conditions (supply voltage, temperature and antenna loading). Therefore, o
ne of the most important
factors in an adaptive predictor is fast convergence rate. The mapping predistorter has the advantage
of performing any order of non
-
linearity and any modulation technique. On the other hand, a major
drawback with the mapping predi
storter is the large size of the look
-
up table to obtain an acceptable
accuracy, which results in long adaptation time. A polynomial
-
based predistortion with adaptive
capability can be a solution. Another limitation to the predistortion approach is that th
e HPA must
still be operated with a substantial back
-
off in order for predistortion to be effective.

7

Channel coding techniques

7.1

C
hannel coding

Digital data transmission channels are subject to various impairments including noise, distortion,
and inter
ference. Therefore, the output of a channel differs from its input and decoding errors can
result from impaired transmission. Although we can achieve the required performance by simply by
using sufficient power, in many cases error
-
control techniques can p
rovide the required accuracy
with less energy.

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Error control techniques can be classified into two main categories: forward error correction (FEC)
and Automatic Repeat reQuest (ARQ). In FEC, redundant bits are added to the information data at
the transmitt
ing end and, by utilizing that redundancy, an attempt is made to correct any errors that
may have been contributed by the channel at the receiving end. On the other hand, in ARQ,
the

receiving terminal does not attempt to correct the errors, but attempts t
o detect them. In the
event that an error is detected, the receiver simply requests the re
-
transmission of the data.

Contrasted with source coding


where source data is compressed by removing redundant
information


FEC coding, which is often called channel coding adds redundant information to aid
in error correction. The purpose of channel coding is to average the effe
cts of channel impairments
over several transmitted bits. In order to achieve this purpose, the channel encoder transforms the
information sequence into a longer binary
-
coded sequence by adding redundant or parity check
symbols.

Channel coding can be clas
sified into two types: block coding and convolutional coding.
The

channel encoder adds redundant bits according to the coding method used. The channel
decoder transforms the received sequence into an estimated information sequence. Since channel
impairment
s may cause some decoding errors, the channel decoder must be implemented in a way
that minimizes the probability of decoding error. This is done by using statistical information about
the aforementioned channel impairments.

With block codes, algebraic pro
perties are very important for constructing good classes of codes
and developing decoding algorithms. Therefore, the theory on decoding techniques for block codes
is quite well developed to the point where decoding techniques based on algebraic calculation

are
usually processed quickly, and thus are computationally efficient. However, most conventional
(i.e.

non
-
iterative decoding) techniques for block codes require the incorporation of hard decisions,
and it is generally difficult to incorporate soft decis
ions. However, LDPC codes, being a special
class of block codes, are an efficient means of providing performance approximating the Shannon
limit, by using iterative soft
-
decision decoding.

Channel codes can also be classified as either systematic or non
-
sy
stematic codes. In a systematic
code, the information bits


called systematic bits


compose part of the codeword, with the rest of
the code bits being called parity bits. Any linear block code can be put into systematic form.